Rep. Eric Swalwell’s Democratic and Republican challengers are still waiting to see which one of them will make it into the top two from Tuesday’s primary and advance to challenge Swalwell head-to-head in November.

As of Thursday afternoon, Alameda County Republican Party vice chairman Hugh Bussell of Livermore led state Senate Majority Leader Ellen Corbett, D-Hayward, by 600 votes — about 1 percent of all votes cast in the race.

But voter registrars in Alameda and Contra Costa counties still had tens of thousands more ballots to count.

Corbett was enduring the primary cliffhanger stoically Wednesday.

“I called Mr. Swalwell this morning and wished him well and congratulated him in coming in first,” she said. “But we’re just going to have to wait for the votes to be counted … We’ll see how that goes.”

Bussell agreed. “I’m guardedly optimistic. At some level, it’s just fantastic we’re even at this point.”

Either way, Swalwell, D-Dublin, seems to be sitting in the catbird seat, having taken 491 percent of the vote.

The district’s voters are 48 percent Democrats, 22 percent Republicans and 21 percent nonpartisans. The freshman Swalwell built a Democrat-nonpartisan bloc — with a few Republicans too, probably — to dominate the middle of the field and crowd Corbett to one side. By drawing almost half the vote, he left her no room to maneuver beyond her liberal, labor-backed Democratic base.

Bussell seemed happy Wednesday and rightly so. His share of the vote slightly exceeded his party’s registration. But few who voted for Corbett will vote for him. And Bussell can expect no monetary aid from the state GOP, the National Republican Congressional Committee or other party entities, who’ll be focused on winnable races. Bussell had about $3,200 cash on hand to Swalwell’s $697,000 as of mid-May.

“It’s a very steep hill to climb,” he acknowledged. “On the other hand, as people take notice that there’s a Republican in the race, they may take a closer look at what I stand for … and Eric might find some of his support falling away as well.”

If Corbett edges out Bussell, it’s hard to imagine Bussell voters pivoting to support her instead of Swalwell — she argues Swalwell isn’t liberal enough for the district. And if her support from labor unions only just barely gets her into the top two this week, it could be hard for them to push her to victory in the larger-turnout, less ideological electorate of November’s vote.